1. Field of the Invention
The present invention relates to a receiver of a mobile communications system, particularly to a received signal strength detector which can extend the dynamic range of the received signal strength and minimize variations in the detection voltage of the received signal strength.
2. Description of the Related Art
Generally, a receiver of a mobile communications system performs detection of a signal received through an antenna. The received signal strength indicator (hereinafter referred to as "RSSI") thus detected is used for control of the transmission output level, setting of a near value and a far value at the time of link setting, hand-off function, etc. which means that it is an element which can guarantee smooth operation of the mobile communications system.
In a receiver of a mobile communications system which performs the above-described operations, a radio frequency signal received from the air through an antenna is supplied to a low noise amplifier through a circulator. The low noise amplifier amplifies at low noise a radio frequency signal supplied through the circulator, and a first mixer mixes and outputs the radio frequency signal, which is amplified at low noise, and a frequency signal, which is generated by a first local oscillator. The signal outputted at this time is a signal at an intermediate frequency zone. An intermediate frequency amplifier amplifies the signal at the intermediate frequency zone and outputs it to an IF processor. The IF processor then receives the outputted signal of the intermediate frequency amplifier, and performs various processing operations to be done at the intermediate frequency zone.
The IF processor is composed of a second mixer, a second oscillator, a second IF amplifier, a limiter, a quadrature detector, an audio signal amplifier, a voltage regulator, a logarithmic amplifier, etc. and performs various processing operations to be done at the intermediate frequency zone. For the IF processor, a frequency modulation IF integrated circuit of Type SA606 manufactured and sold by the Philips Company of the Netherlands may be used typically.
An IF processor composing an earlier RSSI detection circuit and its surrounding circuits are explained in detail on pages 355 through 367 of a data book published by the Philips Company on Oct. 26, 1993 under the title "Philips Semiconductor RF Communications Products.
The IF processor has an RFin terminal and inputs the signal amplified by the intermediate frequency amplifier. The amplitude of the inputted signal is limited to a certain degree by a limiter and is supplied to a non-inverting input terminal (+) of a logarithmic amplifier for detection of the RSSI. The output terminal of the logarithmic amplifier is connected to the RSSI terminal of the IF processor, while an inverting input terminal (-) is connected to an RSSI F/B terminal of the IF processor. A capacitor connected between the RSSI terminal of the IF processor and a ground terminal, a first resistor connected between the RSSI F/B terminal and the ground terminal, and a second resistor connected between the RSSI terminal and the RSSI F/B terminal are surrounding constituent elements which enable detection of the RSSI.
The detection of the RSSI is possible by detecting a voltage Va appearing at the RSSI terminal of the IF processor. That is, once the value of the Va voltage is detected, the RSSI can be detected by a graph of the RF signal level versus RSSI voltage. However, there is a range in which the value of Va voltage is varied as the level of the RF signal is changed as well as a range in which the value of Va voltage is not varied even when the level of the RF signal is changed. The former range is called an RSSI dynamic range. When the RF signal corresponding to the level in this range is supplied to the IF processor, detection of the RSSI is possible from the value of Va voltage. However, if the RF signal corresponding to the level in the latter range, called the RSSI saturation range, is supplied to the IF processor, detection of the RSSI from the value of Va voltage becomes impossible since if a Va of about 3.75 volts is detected, there is no way of knowing whether the level of RF signal corresponding to that voltage is -30 dBm, -20 dBm, or -10 dBm.
In summary, the RSSI dynamic range of the IF processor is limited to a constant range (generally a range of -30 dBm through -110 dBm). Therefore, in case of an earlier receiver of the mobile communications system, the level of the received signal can be detected only when an RF signal of a level corresponding to a -30 dBm through -110 dBm range is supplied to the IF processor.
In the meantime, the earlier mobile communications system is designed to have a low transmission output to minimize interference and a high gain for elements such as a circulator, an LNA, a mixer, an IF amplifier, etc. composed ahead of the IF processor in order to improve receiving sensitivity. For example, in case of a second generation cordless telephone (CT-2) system, it is designed to have the gain of a circulator of -1 dB, that of an LNA of +20 dB, that of a mixer of +6 dB, and that of an IF amplifier of +5 dB (that is, the gain ahead of the IF processor of about 30 dB). However, the gain of the elements ahead of the IF processor should be designed taking into consideration the RSSI dynamic range permitted by the IF processor, and not freely without any restrictions. The higher the gain of the elements ahead of the IF processor, the smaller the range of the RF signal level which can be detected at the IF processor. As an example, when the gain of the elements ahead of the IF processor is designed to be 30 dB, detection of the RSSI is possible if the range of the level of the RF signal transmitted from the transmitter is less than -60 dBm.
On the other hand, to be noted is that the stable range of the level of the RF signal received by an antenna should be more than about -90 dBm taking into consideration the Rayleigh fading phenomena and thermal noise of the receiver which are seen for the mobile communications system of the pico cell concept such as the CT-2 system.
Therefore, taking into consideration the above-described items, the RSSI dynamic range which can be received by an antenna without having a saturated IF processor is merely about 25 dB (-60 dBm through -90 dBm). It can be seen that the RSSI dynamic range of a receiver for the earlier mobile communications system is very narrow. Accordingly, the number of objects of transmitted output which can be controlled by the receiver becomes smaller, and there are interfering signals in the RF signal because of uncontrolled transmitted outputs, which means communications becomes of low quality eventually. In addition, there is a problem of a narrower set range of a near value and a far value which can be used effectively for link setting as the RSSI dynamic range becomes narrower.
The following patents each disclose feature in common with the present invention but do not teach or suggest the specifically recited received signal strength detector for a mobile communications system of the present invention: U.S. Pat. No. 5,390,365 to Enoki el al., entitled Radio Communication Apparatus Having A Received Signal Strength Measuring Function, U.S. Pat. No. 5,193,210 to Nicholas el al., entitled Low Power RF Receiver, U.S. Pat. No. 4,620,114 to Moon, entitled Signal Strength Detector, U.S. Pat. No. 5,430,893 to Myer, entitled Radio Receiver With Increased Dynamic Range, U.S. Pat. No. 5,603,113 to De Loe Jr., entitled Automatic Gain Control Circuit For Both Receiver And Transmitter Adjustable Amplifiers Including A Linear Signal Level Detector With DC Blocking, DC Adding, And AC Removing Components, U.S. Pat. No. 5,487,186 to Scarpa, entitled Automatic Frequency Control Using Split-Band Signal Strength Measurements, U.S. Pat. No. 5,489,868 to Gilbert, entitled Detector Cell For Logarithmic Amplifiers.